1. Field of the Invention
The present invention relates to a corrugated plastic pipe coupling which is used for connection of corrugated plastic pipes having a corrugation such as a helical corrugation, a circular independent corrugation, and a rectangular corrugation and used as pipes for underground electric cables or sewer pipes, and which can easily constitute a connection having an excellent watertightness.
Conventionally, pipes for underground electric cables or sewer pipes are mainly constructed by burying Hume pipes. However, it is difficult to perform piping with various types of conventional pipes since they do not have flexibility, they are short and thus require many connection points, and they are heavy. In order to eliminate these drawbacks, various types of plastic pipes including corrugated pipes having a helical or independent corrugation, sine-wave or rectangular-wave form in the longitudinal section (hereinafter called "corrugated pipes"), are increasingly used year by year since they have flexibility and are lightweight and long.
According to the major demands for a plastic corrugated pipe used as a pipe for underground electric cables or a sewer pipe, such a pipe must have a deformation ratio of the outer diameter of 3.5% or less and a bending characteristic of 100 kg/m.sup.2 or less when compression load P of 0.87R kg/cm (where R=(outer diameter+inner diameter)/4) is applied. The connecting portion must have excellent watertightness for the following reasons. When the pipe is used especially as a pipe for underground cables, the cables generate heat to increase the ambient temperature, and thus the pipe temperature reaches as high as 80.degree. C. in an extreme case. In wintertime, the outer temperature is sometimes as low as -20.degree. C. In this case, the pipe connecting portion receives an external water pressure by the underground water in a heat cycle between -20.degree. C. and 80.degree. C.
A pressure-type drain pipe receives an internal pressure of maximum 5 kg/cm.sup.2 for a long period of time. Therefore, the pipe connecting portion of the pressure-type drain pipe must have excellent watertightness.
FIG. 1 shows the connecting structure of a conventional corrugated plastic pipe having a helical corrugation as a pipe for electric cables. More specifically, in this corrugated pipe, corrugated sleeve coupling 11 having a substantially constant helical corrugation is screwed to bridge opposite ends of corrugated pipes 12 and 13 to be coupled. Tapes 15 are respectively wound on the two ends of sleeve coupling 11 with pressure. In other words, as shown in FIG. 1, sealing tapes 14 are respectively wound on the two ends of screwed sleeve coupling 11 with pressure, and rubber tapes 15 are widely wound with pressure to sufficiently cover tape 14. Each PVC tape 16 is helically wound on one end of each tape 15 with pressure to cover it.
Since corrugated plastic pipes 12 and 13 are flexible, they can be laid and installed in a wound state, and they can have a long length. However, since they are handled in a wound state, their end portions tend to be curled. Therefore, sleeve coupling 11 having a helical corrugation generally has a larger inner diameter than the outer diameter of pipes 12 and 13. Otherwise, coupling 11 cannot be threadably engaged with pipes 12 and 13. As a result, conventionally, the following problems occur and the resultant connecting portion cannot have excellent watertightness.
More specifically, with the connecting structure described above, it is difficult to tightly wind a tape on the corrugation of the pipe, and it is difficult to impart good watertightness to the connecting portion of the two pipes. Since the tapes are exposed to the outside of the pipe, they can be damaged during pipe installation, and the performance of the tapes can be degraded. If a pipe is used for electric cables, when a power is supplied to the electric cables in the pipe, heat (maximum of 80.degree. C.) is generated and sealing tape 14 is fluidized. Fluidized tape 14 may flow into the pipe to further degrade the watertightness.
According to another method, split mold-type clamping couplings are mounted on a pipe through a seal member having tackiness, thus constituting a pipe connecting portion. In this case, however, the seal member of such a pipe connecting portion may cause cold flow, thereby degrading watertightness.